Preface

•Have provisions been made to deal with sulfuric acid dew point corrosion in heat-

ing surfaces while firing high-sulfur fuels or handling flue gas containing SO2 and SO3?

•While firing fuels containing ash or low-melting point solids, can the boiler front end withstand potential slagging and high-temperature corrosion?

•Has the HRSG supplier evaluated the economics of pinch point and approach point while arriving at the HRSG size and steam generation?

•Has the boiler supplier considered all heat sinks available in the plant and tried to maximize energy recovery? The plant engineer has a major role to play in this matter.

•Why is a dual-pressure HRSG offered? Can a single-pressure HRSG not do the same job?

•Why surface areas in finned heating surfaces vary so much between HRSG designs, and how to rationalize difference?

•Significance of heat flux in steam generators and its relation to circulation.

•Relation between superheater tube wall temperature and reduction in its life.

•Is the steam-side pressure drop reasonable at the design point considering all operating loads?

•Advantages of a given design over another, say insulated/refractory lined furnace wall over membrane wall furnace design or a convective superheater versus a radiant design in steam generator.

•Has the performance data at various loads and with different fuels been provided?

•Has complete geometric data of the boiler heating surfaces such as furnace, superheater, evaporator, economizer, and air heater been provided so that an independent evaluation of performance can be done before the boiler is purchased? Many boiler suppliers do not provide this information, and many plant engineers do not know that they should have this important data.

•Will the Kalina cycle be a better choice for their low-gas temperature heat recovery application rather than a steam system?

•Have design features to minimize the boiler or HRSG startup time been provided?

If issues such as those mentioned are not discussed or reviewed before the boiler is purchased, many problems related to thermal performance can surface during operation later on; these include issues such as

•Steam generation much lower than predicted (HRSGs and waste heat boilers)

•Superheater tube failure or overheating and sagging of tubes

•Steam temperature not achieved at rated load

•Flow stagnation in superheater tubes due to low-friction loss and high-gravity loss at low loads

•Too much spray water than envisaged while controlling steam temperature

•Circulation problems resulting in evaporator tube failures

•Steaming in economizer (in gas turbine HRSGs)

•Lower efficiency in steam generators than guaranteed

•Fuel consumption in supplementary fired burners higher than expected or predicted or higher firing temperature in HRSGs than predicted

•Longer startup time than predicted

•Operating costs too high and not envisaged

Plants also have to face several issues regarding boiler thermal performance:

•Unsure whether fouling on steam side or improper design caused tube overheating.

•Unsure whether fouling on gas or steam side caused lower steam output.

•Inability to understand the performance characteristics of steam generator and HRSG versus load and how to optimally load them to maximize plant steam generation and minimize fuel input.

•If operating parameters of a HRSG such as steam generation and exit gas temperature are different from those in the proposal, how to determine if it is due to variations in gas inlet conditions or if it is due to improper sizing of the boiler?

•If a steam generator is operating at say 70% capacity for which performance data such as efficiency or steam temperature or exit gas temperature was not provided by the boiler supplier, how does one find out if it is operating as it should?

When the plant engineer brings the operating problems to the attention of the boiler suppliers, naturally they defend their design and extraneous reasons are blamed for the problems. In my experience not many plant engineers have been familiar with heat transfer and thermal performance aspects, and as a result accept any report from boiler suppliers and as such, are unable to challenge the findings with calculations or studies they have done independently, at the same thing realizing that the design is really flawed. With numerous real world examples on steam generator and HRSG design and off-design performance aspects and calculation modules for major boiler components such as superheater, evaporator, air heater, economizer, and furnace (unfired and fired) given in this book, any plant or process engineer should be able to improve his knowledge in this field and become a competent thermal engineer or consultant and be able to question the boiler supplier if a compromised design is offered and correct the deficiencies before it is manufactured and not after the boiler is installed.

The book provides information on thermal design and performance aspects of steam generators and HRSGs, discusses recent developments in boiler technology, novel design ideas, recent trends in their design, and also how plants can save operating costs before the boiler is purchased. The emphasis is on thermal design, and not on manufacturing issues or mechanical design or stress analysis for which there are other excellent books. Several examples on typical performance-related problems faced by plant engineers with steam generators and HRSGs are completely worked out. At the request of many engineers, the metric and SI system has been followed in this book and wherever possible British units are shown in parentheses. Important heat transfer and pressure drop correlations are presented in all three units. Plant engineers with a background in process or heat transfer can go through them and develop calculation modules or computer codes to evaluate thermal performance of boilers and their components independently, without the aid of boiler suppliers. This I feel will be very helpful to plant engineers in the long run. After reviewing the numerous examples provided in the book, plant engineers can evaluate the performance of a steam generator starting from the furnace all the way to the economizer exit or

the other way around; they can evaluate the off-design performance of a complete HRSG at any gas inlet condition or check the superheater tube wall temperature or its duty based on the geometric data provided, or an evaporator design considering the circulation system proposed, or check the fuel guarantees offered by boiler suppliers for steam generators or HRSGs. For off-design performance calculations, two methods have been provided and illustrated through several examples. Plant engineers can simulate the performance of their operating boilers and perform what if analysis and gather more information about existing units. The adage that knowledge is power is true in any field of endeavor and applies equally to the field of boiler design and performance evaluation. The problems that many plants face today with boilers and waste heat recovery systems are mainly due to the fact that the designs are accepted as such without being critically evaluated before they are purchased. If the plants do not have the resource to perform this evaluation, they may hire an independent consultant to do so. Remember that a plant has to use a boiler for over 30 or 40 years and time and money spent in the evaluation (before the boiler is purchased!) will pay dividends later in the form of low maintenance or repair costs or problem-free operation. When problems such as tube failures, overheating of tubes, lack of performance, and lower than predicted efficiency surface after a few months of operation, the boiler supplier always defends his design, and if the plant does not have knowledgeable thermal or process engineers to counter the supplier’s claims with appropriate calculations and studies, the problems may never be fixed or resolved. I have seen some boilers, in some plants, which should have never been bought as supplied, because the plants spend a lot of time in shutdowns, repairs, and are unable to operate the boiler at the rated capacity.

Chemical plants, refineries, cement plants, cogeneration systems, and power plants invest a lot of their resources, time, money, and manpower in steam generators and waste heat boilers, as steam is the most important working fluid for power generation and process needs. The management expects these steam generators or waste heat boilers to perform well for several decades without problems so that plant engineers can concentrate on their main line of business—whether chemical manufacture or oil refining or power generation. However, this is not the case in many plants. I became an independent boiler consultant a decade ago, and I have had the opportunity to visit several plants worldwide facing severe boiler and HRSG problems. As a boiler designer, I can appreciate the good features in any boiler design offered at the proposal stage as well as spot poor design features or potential problem areas in design. However, in many cases, the boilers are purchased based on the specifications developed by the plant or a consulting firm engaged by them, which in a vast majority of cases does not have specialists in thermal design or performance aspects; the emphasis seems to be on hardware or nonpressure parts. Plant engineers and consultants are familiar with mechanical and electrical aspects of boilers such as instrumentation, controls, fans, pumps, insulation and refractory selection, materials of construction, welding, and erection issues. In fact, the boiler specification developed by their hired consultant will run into hundreds of pages on these topics with hardly a page or two on thermal design or performance aspects. Also, when a boiler is being purchased, engineers typically raise queries on these topics with which they are well versed in but not on thermal design as it is a subject that many are not familiar with! It is some form of Parkinson law perhaps! You discuss things you are familiar with and ignore topics such as heat transfer or process calculations with which you are not familiar. As a result of this, compromised designs are often purchased by plants, resulting in problems such as those discussed earlier. One may wonder if boiler suppliers are untrustworthy. The point is that a busy boiler company may have supplied an off-the-shelf boiler for an application without considering the special needs of the plant, they may have asked their apprentice

engineers to design the boiler, or passed on the drawings of a similar boiler designed years ago without checking the suitability for the present situation, the plant engineer may not be aware that the boiler has very high operating costs, or the customer may not have provided all the information regarding plant parameters or their requirements for future operation such as changes in steam parameters or fuel data. Note that plant engineers have to be more cautious and responsible than boiler suppliers or their consultants before purchasing the boiler as they have to use the unit for 30 to 40 years, while for the consultants or boiler suppliers it is just one more boiler and they move on to other projects once the boiler or HRSG has been sold! Hiring a consultant who specializes in process heat transfer or thermal engineering can help a plant foresee problems in their boiler before it is purchased and save them a lot of money in the long run. Alternatively, plants should develop a team of in-house engineers who specialize only in boiler-related process calculations and who can check the design offered by the boiler supplier; the purpose of this book is exactly that, namely, to educate plant engineers and consultants in thermal design and performance calculations. Many reputable boiler suppliers while making a sales pitch say that they have thousands of such boilers and similar designs in operation and yet within a few months of operation, several problems cited earlier surface and the plant engineers run from pillar to post to fix the issues or they may never be fixed as the design modification would be as expensive as replacing the unit!

When I was a boiler designer in the United States, custom-designed boilers were offered by my company as these have several advantages, as discussed in Chapter 3, over standard or off-the-shelf designs. Standard or off-the-shelf boilers are predesigned for a particular steam output with tube geometry, tube spacing, height, width, length all fixed. In fact, they may have already been built and branded as, for example, 40 t/h or 70 t/h boilers. Note that the efficiency, steam temperature, gas pressure drop through the boiler, or fan power consumption is dependent on the quantity of flue gas flowing through the boiler. As explained in Chapter 3, with the introduction of flue gas recirculation (FGR) for NOx control, one has to check the boiler performance based on actual flue gas flowing through the unit and not select a boiler from a catalog based on steam capacity. Local conditions such as site elevation and ambient temperature may also affect performance. A few plant engineers even today order a water tube boiler based on the model number. While such an approach may save time and money, it is suitable wherever emission regulations need not be complied with or when saturated steam is generated or when clean, standard fuels are fired. When the boiler has to comply with emission regulations or when special fuels are fired, about 15% excess air and 15%–30% FGR may have to be used depending on the NOx levels desired at that particular location; the flue gas mass flow would be about 20%–40% more compared with a boiler without FGR and hence the back pressure will be at least 40% more, calling for a larger fan for the same steam output if the standard boiler is used. The boiler exit gas temperature will also be higher due to the higher flue gas mass flow, resulting in lower boiler efficiency as shown in Chapter 1, where the effect of excess air and exit gas temperature on boiler efficiency is discussed. Hence, based on actual excess air and flue gas flow, the furnace dimensions, superheater surfacing, convection bank tube spacing, boiler height, and economizer surface, to mention a few factors, have to be reviewed and adjusted to obtain a low back pressure and a high efficiency; in fact, I would recommend performing such a detailed evaluation for every boiler you purchase; Chapter 3 shows an example of how this is done. This re-engineering of the boiler on a case-by-case basis results in an efficient design with a smaller fan. So when a custom-designed steam generator was proposed for a project, the consultant who was evaluating various offers did not appreciate the fact that the boiler was custom-designed for the specific requirement and that it was

not a standard model; he was questioning why our forced draft fan was smaller than that of the competition! He had also come to the conclusion that our fan will not work! I had to then explain to him about custom designing of boilers and the advantages accrued to its owner over a period of time and how our tube spacing and tube lengths were adjusted on a case-by-case basis to lower the gas pressure drop and maintain the efficiency considering the higher flue gas flow through the boiler arising out of local NOx and CO regulations! Many large boiler suppliers do not want to change the boiler design or dimensions on a case-by-case basis as it is expensive, while a small boiler company can make these changes easily as this does not need approval from the corporate office.

To give another example, many plants even today purchase boilers or HRSGs based on surface area, which is a big mistake! Engineers tabulate surface areas offered by different vendors and if the prices are comparable, select the vendor with the largest surface area, and to add insult to injury, the customer thinks he is getting a bargain! As explained in Appendix E, surface area of a finned tube superheater or evaporator in a HRSG can vary by about 50%–100% for the same duty depending on the fin geometry chosen. Higher fin density leads to higher heat flux inside the superheater tubes, resulting in higher tube wall temperatures and shortened superheater life. Higher fin density also results in a lower overall heat transfer coefficient, necessitating a larger surface area for the same duty and higher gas pressure drop. The customer was under the impression that our superheater was too small as the surface area with our small fin density was about 60%–70% of that of the competition and hence would not achieve the steam temperature. Fortunately, the customer called and asked me this question and I sent him copies of the articles I had published on finned tubes explaining how the U values decrease with an increase in ratio of external to tube internal surface and the product of U × A (overall heat transfer coefficient × surface area) was the important thing to check and not A alone. (The problem is many boiler vendors will not provide information on U values but just the surface area, and unsuspecting engineers may evaluate the options based on surface area alone.) I explained to him about the increase in heat flux caused by poor selection of fins and how the tube wall temperature would increase and as a result how tube life would decrease. After this presentation, they were convinced and appreciated the importance of proper selection of fins in HRSGs. Plant engineers who are not process savvy will jump at the offer with the highest surface area and may have to deal with issues such as high heat flux inside tubes or overheating of tubes later! The book gives a few examples of the effect of fin geometry on the overall heat transfer coefficient and tube wall temperature in the evaporator and superheater so that plant engineers or consultants are not mislead by surface areas when buying a HRSG. Bottom line is “Don’t go by surface area A alone. Ask for the U (overall heat transfer coefficient) for each heating surface!” Then check U × A, which should be the same for all vendors for any component with the same duty. The appendices also provide methods to evaluate heat transfer coefficients inside and outside plain and finned tubes so that U may be estimated independently for each heating surface and checked with data provided by the boiler vendor. There may be some differences in U values between boiler suppliers, but they should be small.

Many boilers are purchased based on initial cost without any consideration of operating costs arising from high gas pressure drop or lower efficiency or maintenance costs. For example, a refractory lined boiler will have higher maintenance costs associated with its startup and operation compared with a completely water-cooled furnace as discussed in Chapter 3. The cost of fuel and electricity has increased over the years and ignoring them in the evaluation would not be correct. Examples are provided in the book on evaluating gas pressure drop or selecting HRSGs with low pinch and approach points and low back

pressure, which may be slightly more expensive but pay dividends in the long run in the form of lower operating costs and fuel consumption if the HRSG is fired. Methods to improve the efficiency of a steam generator or HRSG using secondary heating surfaces or by lowering the feed water temperature or by heating air using a closed loop glycol system are discussed.

When fuels with potential for acid dew point corrosion are fired in steam generators, some consultants suggest that the exit gas temperature be raised above the dew point. However, this does not prevent acid vapor condensing at the feed water inlet section of the economizer tubes, which may be operating below the acid dew point temperature. In fact, the temperature of the feed water should be raised to avoid condensation and consequent local corrosion and not the gas temperature alone as the tube side heat transfer coefficient is much higher than the gas side coefficient and hence governs the tube wall temperature. Raising the gas temperature above the dew point may prevent corrosion in the ducts downstream of the economizer but not in the tubes per se. One should also understand the fact that as the steam generator load decreases, the economizer exit gas temperature will also decrease and the ducts also can get corroded at part loads. This can be prevented by ensuring that the feed water temperature is above the acid dew point as discussed in Chapter 3, as a result of which the exit gas temperature will always be above the dew point at all loads! The book also discusses the option of using condensing economizers and its sizing procedure and evaluation of additional energy recovery and materials to be used in such applications. The importance of keeping a boiler warm during startup and shutdown while using sulfur-containing gases in boilers and HRSGs is also stressed. Examples show how one may estimate the cold end tube wall temperature in the air heater and economizer and also the effect of the counter flow versus parallel flow arrangement on cold end tube temperature with plain and finned tubes.

When specifying gas turbine HRSGs, consultants not familiar with process calculations or HRSG temperature profile analysis expect a certain exit gas temperature from the boiler or a certain amount of steam generation in the unfired mode. This may not be always attainable as the gas–steam temperature profile is dependent on the pinch and approach points chosen and the steam pressure and this is a thermodynamic limitation. The higher the steam pressure, the higher the exit gas temperature unless one adds a low-pressure steam system. Thus, one cannot predict a particular exit gas temperature without doing an analysis of temperature profiles. Also, many plant engineers are not aware of the fact that supplementary firing is the most efficient way of generating additional steam in a HRSG. If a refinery has both steam generators as well as gas turbine HRSGs, it is prudent to plan future HRSGs as fired units so that steam can be generated at more than 100% efficiency compared with about 93% (on lower heating value basis) in steam generators for gas firing. This point is explained in Chapter 5 on HRSG simulation. The cost of a HRSG is mainly a function of the exhaust gas flow through it, and since this remains the same whether the HRSG is unfired or fired, planning the HRSG for firing in the future is a good idea as the additional cost of burners and insulation will be minimal. On the other hand, the plant can obtain 100%–300% more steam through supplementary firing or furnace-fired HRSGs.

HRSG simulation is explained in Chapter 5. Without designing the HRSG per se, plant engineers can obtain valuable information on steam generation, burner duty, gas–steam temperature profiles, and off-design performance. Using this concept, it is also possible to obtain the efficiency of single-pressure versus dual-pressure HRSGs without designing the HRSG per se. One may also rearrange the boiler components to optimize energy recovery. Anyone familiar with energy balance calculations can perform this analysis. Often, consultants suggest a dual-pressure HRSG without doing a temperature profile analysis.

Dual-pressure HRSGs are not required for all cases involving HP and LP steam requirement. The ratio of LP steam flow to HP steam and the ratio of LP steam pressure to HP steam pressure impact the temperature profile, and sometimes it is more prudent to go with a single-pressure HRSG and take off the process steam from the drum and pressurereduce it rather than install a dual-pressure HRSG that could be more expensive than a single-pressure unit as shown in Chapter 5. Unless the plant engineer or the consultant is familiar with such analysis, a dual-pressure HRSG may be specified by the plant or the consultant without any analysis, which can be expensive in the long run. It is my desire that a consultant or a plant engineer review any boiler or HRSG design offered to a plant and independently check if there is scope for improving the energy recovery.

Often HRSGs operate in the fired and unfired mode. Common practice is to locate the superheater downstream of the burner and design the superheater in a way that steam temperature is obtained in the unfired mode and then the stream is desuperheated in the fired mode to obtain the desired steam temperature using demineralized water spray. This design is not expensive as the superheaters are built as a single module. This approach, however, may increase the tube wall temperature of the superheaters in the fired mode; also some plants may not have demineralized water for attemperation. One novel approach discussed in the book is to split the superheater so that the final superheater is located beyond the gas turbine and the primary superheater is located downstream of the burner. This approach also lowers the tube wall temperature of the superheaters and eliminates the need for spray attemperation in many cases.

Low-temperature heat recovery as in cement plants offers many challenges. If steam is used as the working fluid, pinch point limits the heat recovery potential as the lower the inlet gas temperature, the higher the exit gas temperature unless there is multiple pressure steam generation. (The reason for this is explained in Chapter 5.) The Kalina cycle HRVG discussed in the book enables the exit gas temperature from the HRVG to be much lower than possible with steam systems due to the varying boiling point of the ammonia–water mixture working fluid. Gas inlet temperatures of 250°C–350°C are ideal heat sources for the Kalina cycle.

There are applications where space is a premium. In one project, a plant had to fit a steam generator of 50 t/h capacity inside a building of width 3.5 m. For example, in Chapter 3, a steam generator with a width of about 3 m is shown with finned tubes in the convection section. A conventional A- or D- or O-type boiler design would not have fitted into this space as its width would have exceeded 5 m and considering space around the boiler, it would have been impossible for a regular or conventional boiler to be installed. As a boiler designer, I came up with an idea of using a O-type boiler with finned tubes with a width of less than 3 m; to reduce the total boiler length, finned tubes were used in the convection bank. The plant engineers and consultants involved in the project appreciated what we were offering and hence the project was a success. If the plant engineers had not been open to this idea, they may have been without a boiler or destroyed the building and installed a standard boiler! It was explained to the plant personnel that finned tubes are used in HRSGs and the boiler offered simply borrowed that concept though it was the first time this was done in a steam generator!

Chapter 3 also discusses an elevated drum steam generator with external downcomers and risers. This concept helps one to shop assemble large capacity steam generators and minimize field work; only the drum with associated downcomer and riser piping is assembled in the field. This approach saves a lot of time and money for the end user. This concept is widely used in HRSG design, but it is new to steam generators. As plant engineers who could appreciate the circulation system were involved in the evaluation

and purchase of this boiler, it was not difficult to sell the idea. Hence, it is important that engineers with a background in thermal design or process calculations are also involved in the purchasing decisions of a plant.

In package oiland gas-fired steam generators, many boiler vendors offer a radiant superheater even if the steam temperature required is only about 350°C–400°C. It is possible to attain about 500°C with a convective superheater, which has fewer tube failures or performance problems compared with the radiant design. The radiant superheater is more prone to failures at full as well as part loads compared with the convective design, as discussed in Chapter 3. Another common problem is that the flame may impinge on the superheater if the burner is not properly adjusted and there is always a dispute whether the burner is causing the tube failures or if the design of the superheater itself is flawed. Another error made by some consultants is to specify that the steam pressure drop in the superheater should be low; I have seen figures as low as 0.35 kg/cm2 (5 psi) at full load. This is a very low value for a good design, particularly if the degree of superheat is large and the boiler has to operate over a wide range of steam flows. One has to review the steam velocity, the tube wall temperature, and the pressure drop at all loads. As a designer, I used to ask them about the flow distribution issues at lower loads with such a low pressure drop to start with. At 50% load, the pressure drop will be less than 0.1 kg/cm2, leading to flow stagnation or reverse flow in some tubes with downward flow where the gravity head opposes the friction loss; hence, overheating or failure can occur if the gas temperature is high. The importance of tube side pressure drop in superheater and the significance of low flow in upward and downward paths are discussed in Chapter 3. Lack of understanding of process or thermal design is the reason for such ridiculous specifications. Also, if the superheater fails because of the low pressure drop suggested by the consultant, then who is at fault?

Some consultants while evaluating offers from different boiler suppliers do not even obtain the thermal or tube geometry data for the boiler surfaces, which is required to do a performance evaluation at a later date if need arises. The plant engineers feel that the consultant who has prepared the specifications will do a good job and do not interfere with the evaluation process. The consulting company may or may not have thermal or process engineers on their team. If they do not, then the plant may have to be at the mercy of the boiler supplier when a boiler problem arises later on, when they do not even have the minimum information to be able to evaluate the boiler’s performance. As a boiler consultant, I have seen this problem in many plants and hence one of the services I provide is to evaluate boilers from thermal and performance viewpoints and ensure that all tube geometry data for steam generators and HRSGs are made available by the supplier before purchase. Chapters 3 and 4 present several examples of the format of the tube geometry data. Plant and process engineers should ensure that suppliers of boilers for steam generators and HRSGs provide information in this format so that future performance evaluation becomes easy. Presently many boiler suppliers get away with very sketchy information on their boiler tube geometry and performance details. Only when a problem arises in the boiler and the performance has to be checked do plant engineers run around looking for basic information about the boiler. The boiler supplier may not even be around when these data are required!

Consultants sometimes make the mistake of demanding certain margins over surface areas of various boiler components. This affects the performance of components located downstream. The misconception is that a 15% margin will provide about 10%–15% more duty! An example is worked out to prove that it is not so and this method of specifying boilers can cause problems in performance, particularly if several heat transfer surfaces are included in the design.

Many plant engineers are not aware of the differences in boiler efficiencies based on higher and lower heating values. It is the practice in the United States to state boiler efficiency on an HHV basis, whereas in Europe and Asia, it is based on LHV. The LHV efficiency is higher by a factor equal to the ratio of higher to lower heating value of the fuel. Hence, unless the basis of efficiency is mentioned, clearly evaluation can be skewed. Also, while specifying waste heat boilers, consultants sometimes provide the volumetric gas flow instead of mass flow. This practice should be avoided. One should provide the flue gas flow in mass units such as kg/h and the flue gas analysis as the gas properties and duty are affected by the gas analysis. Energy balance is done using mass of flue gas and not its volume. Variation in steam output can be even 5% or more if some boiler vendor assumes an analysis based on his experience while another vendor assumes a different gas analysis and computes the density. I have seen n different answers from n engineers for the density of flue gas at a given pressure and temperature!

Often performance information on steam generator or HRSG is not completely provided by the boiler supplier when a boiler is sold; little technical or process information is offered during the contract stage, but shrewd plant engineers should ask the boiler supplier to furnish more information on thermal performance such as part load performance and what happens if some steam parameters change such as steam pressure or feed water temperature or fuel analysis. Some plants may operate at say 25% load for most of the time and then go on to 100% load. The plant may need the specified steam temperature at 25% load also, which may not have been discussed. Once the boiler is started up and if it is found that the steam temperature is much lower, the boiler supplier cannot be blamed. Sometimes, a low BTU fuel may be fired after a few years of installation. This will increase the flue gas flow through the boiler and also affect the furnace absorption, steam temperature, backpressure, and efficiency. This scenario must be discussed with the boiler supplier before the boiler is purchased. The plant can make a decision whether to buy the larger fan now or later, when they switch to the low Btu fuel firing mode.

A steam plant was planning a large, new waste heat boiler; it was generating saturated steam in another waste heat boiler and wanted to superheat the saturated steam in the new boiler. This type of design calls for a large superheater and one has to see what happens to the steam temperature and the tube wall temperature when the import steam is absent. A large amount of spray would also be required to control the steam if import steam is absent. The superheater has to be designed considering both the cases. Refineries often encounter such situations as they have numerous process streams generating saturated steam and they would like to superheat this in another new boiler whether it is a steam generator or waste heat boiler. Separately fired superheaters may be inefficient and prone to problems compared with convective-type superheaters in steam generators. In solar plants that are combined with waste heat boilers or HRSGs, the saturated steam from the solar panels must be superheated in the waste heat boiler. Even when the solar panel does not generate steam, the superheater of the waste heat boiler should operate safely without overheating the tubes.

Another example of creativity in boiler design is discussed in Chapter 3. A plant wanted a steam generator to operate at a low pressure for the first few years and then at a higher pressure. Boiler vendors suggested superheater designs that had to be replaced in future as steam-specific volume decreases with pressure and the same design cannot be used while maintaining the steam capacity. Chapter 3 describes the concept used in the design offered, which made them choose the same superheater and operate the boiler at the same steam capacity at both steam pressure levels by simply changing the inlet and outlet steam connections. The customer understood our process calculations and went with our design concept. Other options would have been very expensive for the plant.

Often gas turbine HRSGs and waste heat boilers are designed for a particular gas inlet flow and temperature; however, in operation, the exhaust gas conditions may be different from the design. How can one tell if the HRSG performance is satisfactory or not? The HRSG supplier blames the drop in gas turbine exhaust gas flow or temperature as the reason for the poor HRSG performance, while the gas turbine supplier blames the HRSG design. The plant is caught between them. To minimize such issues, the HRSG supplier may be asked to provide data on the HRSG performance for various gas inlet conditions. With such a performance chart, the plant engineer knows what he can expect from the HRSG at different gas inlet conditions. Problems arise when such data are not sought before the HRSG is purchased. If an HRSG designed to generate 50 t/h of steam at 450°C in the unfired mode generates only 45 t/h of steam at a slightly lower steam temperature, plant engineers scramble around to find out why. The book gives an example on how to perform off-design performance calculations and ensure that the HRSG design is satisfactory at any operating point. Plant engineers may then challenge the HRSG supplier if they feel that the HRSG is not adequately designed.

Circulation is another important issue in boilers. If downcomers and riser pipes are not properly located or sized, circulation can be hampered. There can be reverse flows in evaporator tubes or stagnation if downcomer tubes are heated by flue gas. This point is discussed along with methods to estimate circulation in fire tube as well as water tube boilers in Chapter 2. Correlations for departure from nucleate boiling (DNB) are also given along with estimation of actual and allowable heat flux in evaporator tubes.

Thus, the book is aimed at plant engineers, process engineers, and consultants who want to understand and perform boiler thermal calculations and evaluate the performance of new or existing steam generators and waste heat boilers and become more knowledgeable buyers or plant engineers. Recent developments in boiler design and industry trends are addressed. It is hoped that this book will be a good reference book for plant engineers, consultants, and even boiler designers!

Problems have been worked out in metric units, and SI unit values are shown alongside in several examples; British units are also shown in parentheses for important data, formulae, and results; though SI units are used in college textbooks in the United States and in other parts of the world, engineers in the boiler industry in the United States still work with charts and data in British units; boiler companies in South America and the eastern part of the world such as Malaysia and Indonesia still work with metric units. Many boiler companies have yet to convert their design standards and charts to SI units and hence this approach. Important correlations throughout the book have been provided in tabular form in all three units so that plant engineers may quickly check the results using whichever system they are familiar with.

The starting point in the design of any heat transfer equipment is the analysis of thermal and transport properties of flue gas. This is obtained from combustion calculations and hence we start with this topic. Chapter 1 deals with basic combustion calculations given the fuel analysis and how one may obtain excess air value from oxygen readings in flue gas. Simplified combustion calculation procedures for various fuels help plant engineers arrive at quick estimates of air and flue gas quantities from plant operating data. For performing furnace performance calculations, the combustion temperature of products of combustion is required; estimation of this value for gaseous and oil fuels is illustrated with and without FGR. HRSGs are often supplementary fired, and consumption of oxygen in turbine exhaust as a function of fuel input should be known to process or plant engineers. The ASME heat loss method of efficiency calculation is explained with examples, and simplified equations for estimating boiler efficiency are provided. Conversion calculations

for pollutants such as NOx, CO, UHC, and CO from mass to volumetric units (SI, metric, British) are illustrated as plant engineers should have an idea of the emission levels of various pollutants and the different units used throughout the world. Acid dew point correlations for various acid vapors are also presented so that one can address the problem of low-temperature corrosion.

Chapter 2 describes calculations involving boiler furnace such as estimation of furnace exit gas temperature (FEGT) and furnace duty. The significance of heat release rates and heat flux inside tubes is discussed, as also the view factor determination based on tube diameter and spacing in the membrane wall enclosure, which is helpful for estimating local heat flux inside the tubes. FEGT is estimated by different methods. Information on FEGT enables one to check how downstream components such as superheaters, evaporators, and economizers perform. Distribution of external radiation to the heating surface at the furnace exit is critical if a radiant superheater is used. Recent developments in furnace design such as completely water-cooled furnaces are discussed. Correlations for critical heat flux are provided so that one may check if departure from nucleate boiling is likely. This is followed by circulation calculations.

Chapter 3 on steam generators describes trends in large steam generator designs such as multiple-module, elevated drum design types of boilers such as D, O, and A and forced circulation steam generators. The importance of custom designing and the advantages it offers to the end user are discussed with numerical examples. Pollution regulations limit emissions of NOx and CO and methods used to meet the limits such as the use of SCR (selective catalytic reduction system) or FGR and how these methods impact boiler design and operating costs are explained. The effects of excess air, FGR on boiler performance, and methods of reducing operating costs using custom-designed boilers are discussed. Emission regulations have had a major impact on boiler designs during the last two decades, and these issues are addressed. The significance of the surface areas of various components and how it should not be the criterion for selecting boilers is explained. An efficiency improvement scheme using a glycol heat recovery system behind the economizer that is in operation in a large boiler plant is discussed. How efficiency may be improved using a lower feed water temperature is also explained with calculations for a natural gas-fired steam generator. Advantages of convective over radiant superheaters are explained with calculations of tube wall temperatures of superheaters. Novel designs of steam generators in operation are described so that plant and process engineers may apply these concepts in their plants if need arises. Various methods of steam temperature control are discussed. If a plant engineer goes through the many numerical examples and studies them, he will be able to do similar calculations for the boilers in his plant and even challenge the boiler supplier if some data are different or if the boiler is not performing well!

Performance calculations of a complete steam generator using field data or data provided by boiler suppliers are outlined step by step. Part load calculations are also illustrated. Plant engineers may go through these examples and see how they can apply these methods to their existing units and check if their calculations match the field data. Examples of tube wall temperature calculations for a superheater as well as cold end temperature of plain tube and finned tube economizers also will enable plant engineers to check their existing superheater for overheating issues or the economizer for lowtemperature corrosion potential if any. Any plant engineer familiar with programming codes can convert these calculations into a program and evaluate the performance of their steam generator at any load without support from boiler suppliers, which is the objective of this book. Important data required for such an analysis are also listed so that plant

engineers can obtain these data before buying the steam generator! Reliance on boiler suppliers is minimized by such practical examples!

Chapter 4 on waste heat boilers starts with the classification of waste heat boilers and addresses the importance of flue gas analysis; fire tube versus water tube boilers used in chemical plants, refineries, and cogeneration systems are described; heat recovery in sulfur plants, hydrogen plants, and cement plants and the effect of the fouling factor on performance are discussed. A description of a boiler in a cement plant using Kalina cycle is given. The advantage of this system over the use of steam water as the working fluid in low-temperature heat recovery systems is discussed. Features of unfired, fired, natural, and forced circulation gas turbine HRSGs are described. Combining the operation of steam generators with gas turbine HRSGs for better utilization of fuel is explained. Performance calculations for an operating HRSG are completely detailed so that plant engineers can check if the HRSG or waste heat boiler has been designed adequately based on operating parameters at any load! Consultants often demand a large margin on heating surfaces, and the impact of oversurfacing on steam generation and superheater temperature is shown with an example. Fluid heaters using waste flue gases are often used in chemical plants; heat flux inside tubes and film temperature estimation are an important aspect of this design and hence illustrated by an example. Cogeneration plants would like to take off saturated steam from the drum for process heating or other use and the balance will be superheated. An example of performance calculations when steam is exported from the drum of the waste heat boiler is illustrated. Plant engineers can obtain the steam temperature or superheater tube wall temperature when steam is exported. How emission regulations are impacting HRSG designs by increasing their operating costs as well as making the design more complicated is illustrated. Calculation for economizer and tubular air heater design and off-design performance and cold-end tube wall temperature estimation will help plant engineers foresee problems with low-temperature corrosion.

Calculations for design and off-design performance of fire tube boilers are detailed. Tube sheet temperature estimation with and without refractory and ferrules is included. Several examples illustrate the calculation of heat flux in fire tube and water tube boilers, tube wall temperatures, and film temperatures for heat transfer fluids. One of the important factors in sizing of boilers is the tube size. The impact of tube size on heat transfer coefficients in both fire tube and water tube boilers as well as on their weight and gas pressure drop is illustrated with examples. The advantages of using smaller diameter tubes in fire tube as well as in water tube boilers are illustrated with examples. Often plant engineers would add an economizer to their fire tube boiler to improve the efficiency. An example is provided showing how the revised duty and steam generation may be estimated when an economizer is added.

Information to be considered while developing specifications for a waste heat boiler is discussed at the end of the chapter, particularly the tube geometry data. Plant engineers should obtain a quotation with these data as it will help them analyze boiler performance when needed. Today, in several projects I see that this information is not readily available and plant engineers spend a lot of time trying to obtain this data. In one case, information on streams in a superheater was not clear and the boiler designer was not in business to help the plant engineer. It took a lot of time to inspect the boiler in operation and discuss with the fabricator and review header drawings before an estimate could be made. The number of streams in a superheater or economizer is an important piece of information, and process and plant engineers should have this if they want to evaluate the performance of the superheater. Appendix B discusses the significance of this term.

Chapter 5 describes the HRSG simulation process. This is an important tool for plant engineers planning cogeneration projects using gas turbine HRSGs. Using the concepts outlined, plant engineers can obtain so much information about their future or the operating HRSGs: How much steam can be generated given the exhaust gas flow conditions? How much fuel is required to generate additional steam? Will the economizer generate steam at low loads? How can one evaluate field data and compare it with the guarantee data? Is dual-pressure HRSG really required for given steam conditions or will a singlepressure HRSG do? How efficient is fresh air firing? If in an existing HRSG with a superheater the process steam is taken off the drum, what happens to the steam temperature? These and similar issues can be addressed by plant engineers without designing the HRSG or even without contacting the HRSG supplier. Hence, simulation is a great planning tool. Examples have been provided to illustrate these problems and results compared with physically designed HRSGs to show that the accuracy is reliable. They may use the results from simulation to check the HRSG performance or even its operating data.

Chapter 6 on miscellaneous calculations provides several procedures for calculations or sizing of components associated with boilers. Condensation heat recovery is explained in detail along with estimation of energy recoverable in water vapor condensation and sizing of plain or finned tube condensing economizers. Acoustic vibration analysis, flow in blowdown lines, simplified transient analysis, drum-level fluctuations, insulation and refractory performance calculations, pressure drop in air and flue gas ducts, wall temperature estimation in uninsulated stacks, and sizing of natural convection bath heaters and coils immersed in a boiler drum for cooling superheated steam are also illustrated. Simplified procedures for transient analysis will help plant engineers get an idea of the time to heat up boiler components or see how steam pressure or drum level fluctuates when there are upsets in flue gas flow or temperature or feed water flow.

The appendices provide calculation procedures for basic sizing and performance evaluation of heat transfer components and estimation of heat transfer coefficients inside and outside tubes. Procedures for evaluating flue gas properties are also given. Correlations for heat transfer and pressure drop outside and inside tubes are provided in SI, British, and metric units. The important concept of streams in boiler components is explained in Appendix B. Completely worked out examples on solid and serrated fins in Appendix E will enable plant engineers with programming skills to model economizer in steam generator or waste heat boiler components. Examples illustrate when finned tubes should be used and why they should not be used in some applications and how fin geometry should be selected. With several worked out real world examples for the evaluation of performance of steam generators or waste heat boilers, plant engineers have valuable resources to fall back on when such needs arise.

In conclusion, I can say that I have learnt a lot as a consultant during the last decade from numerous plant problems and performance issues and found that plant engineers depend solely on boiler suppliers when it comes to evaluating a boiler problem or finding a solution to it. It should not be so. Hence, this book has been written with the main purpose of educating plant engineers with a flair for process heat transfer on boiler design and performance calculations. It is my belief that if they study the appendices and go through the numerous practical and real world examples that have been worked out completely throughout the book, they will be able to evaluate the performance of boilers in their plants on their own, challenge boiler suppliers when poor or compromised designs are offered, suggest changes to proposed designs to lower operating costs, and analyze any boiler problem and arrive at the solution themselves or discuss intelligently with the boiler supplier if need be with their findings. Boiler designers will also find useful information throughout the book on design

and performance aspects of various boiler components. Boiler suppliers will not be able to offer simple excuses for poor performance of their boilers as the plant engineers will now have backup calculations to prove their point and be well armed with information.

Worked Out Real World Problems of Interest to Plant Engineers

The book has several worked out examples in metric and SI units dealing with design and off-design performance aspects of steam generators, waste heat boilers (fire tube and water tube) and HRSGs, and their components such as furnace, evaporator, superheater, economizer, air heater, all based on real world examples. Any plant engineer with heat transfer or thermal sciences background at a collagen level should be able to follow these examples and apply the methodology to solve various boiler-related problems in their plant, check boiler designs offered during proposal stage, suggest improvements before the boiler is purchased, and challenge the boiler supplier if there are problems in operation. These examples provide plant engineers with tools and wherewithal to perform various types of boiler calculations, and they need not depend on boiler suppliers for help. They are now empowered to question boiler suppliers on design or performance issues and not accept any design or report from them as such. This is the main objective of the book.

A Few Typical Solved Problems

•A plant receives a quotation from a boiler vendor for a finned tube superheater. Information on process data and tube geometry details is given. Using heat transfer correlations presented in the Appendices, the U is estimated (overall heat transfer coefficient) and the adequacy of surface area is checked in addition to gas-steam side pressure drops and tube wall temperatures. To predict its off-design performance, two methods are discussed, one being the NTU method and the other the conventional method. (A similar analysis for evaporator, economizer, and air heater is given.) These examples will educate plant engineers and enable them to perform similar calculations and rectify any flaws in the component design before it is ordered.

•Using fuel data, furnace dimensions, and tube geometry details of an oiland gas-fired steam generator, the complete performance of a steam generator is evaluated at full and part loads. One example starts from the economizer exit and works backwards all the way to the furnace, and another method starts from the furnace end and works through to the economizer exit. These examples will help plant engineers to check independently if the boiler supplier’s surfacing of furnace, superheaters, boiler bank, economizer, fuel consumption and efficiency is reasonable and as guaranteed.

•An example shows how in a natural gas-fired steam generator the feed water temperature entering the economizer may be reduced to improve fuel utilization and boiler efficiency. The feed water entering the economizer is used to preheat the make-up water. One must ensure that the feed water is cooled to slightly above the water dew point of the flue gas.

•Examples compare results for a HRSG obtained from the simulation process with that obtained using actual tube geometry details to check accuracy of simulation techniques.

•Using a simulation method, examples illustrate if a single-pressure HRSG can be a better choice than a dual-pressure HRSG under certain circumstances. The effect of the ratio of HP (high pressure) to LP (low pressure) steam flow and steam pressure is illustrated. This helps one to arrive at the least-expensive HRSG configuration instead of simply accepting a design from a HRSG vendor.

•An example illustrates how a plant may optimize the configuration of a multimodule HRSG to maximize energy recovery using the simulation method. (HRSG suppliers may not have the time to study these options.)

•A simulation example illustrates how by splitting superheaters so that a portion is located upstream and another downstream of the burner a flat steam temperature may be obtained in both unfired and fired modes and over a wide load range without the need for a desuperheater!

•An example shows how the condensation duty may be estimated when flue gas is cooled beyond its water dew point. Examples also show how plain tube and finned tube condensing economizers may be sized.

•An example shows how a coil located inside the steam drum may be sized for a given duty. An example design of a steam desuperheater coil located inside the steam drum is given.

•Effects of sudden changes in process steam demand or feed water cut-off on drum level and steam pressure are illustrated with examples. Examples also show how one may estimate the time to heat up an evaporator or a superheater using flue gases.

Several more examples on basic and applied heat transfer calculations related to boiler, HRSG design, and performance are provided.

Articles on boilers published by the author can be downloaded from the website http:// vganapathy.tripod.com/boilers.html. The author may be contacted at v_ganapathy@ yahoo.com.